This invention relates to a free flowing powder for flame spray applications.
Free flowing powders for flame spraying have been made by various agglomeration methods which make free flowing powders of normally non-flowing small diameter particles. One such agglomeration method is spray drying. Agglomerates are formed in spray drying by atomizing a slurry of powder, binder and liquid into a drying chamber where the liquid is evaporated. The result is a generally spherical agglomerate held together by the binder. U.S. Pat. No. 3,617,358 describes an agglomeration process using an organic binder.
Other agglomeration processes have been developed to overcome what may be undesirable effects caused by the presence of organic binders. In some cases, the organic binder may cause fouling of the plasma gun due to vaporization of the organic. The presence of organics may even decrease the apparent density of the powder or affect the flame spray coating. In U.S. Pat. No. 3,881,911 to Cheney et al., the agglomerates are presintered to remove the binder. U.S. Pat. No. 3,973,948 to Laferty et al. uses a water soluble ammonia complex as a binder and U.S. Pat. No. 4,025,334 to Cheney et al. uses an aqueous nitrate solution.
Because of their relatively large size and low surface area as compared with the original small particles which are often irregular in shape, the agglomerates have improved flow properties. However, the increased particles size and lower density resulting from agglomeration can be a disadvantage. Hence, plasma densification may be employed to produce spherical, dense, and homogeneous particles. According to this process, the agglomerated powder is entrained in a carrier gas, and fed through a high temperature plasma reactor to melt the agglomerated particles. The melted particles are cooled to avoid coalesence so as to produce spherical dense particles. The use of the dense particle in plasma spray applications can result in a dense, smooth coating which requires little or no finishing by grinding or machining as compared to coatings produced by the agglomerated particles. Further, the densified particles have improved flow characteristics and enable the use of a reduced volume of material and decreased processing time to achieve improved efficiencies in flame spraying. U.S. Pat. Nos. 3,909,241 and 3,974,245, both to Cheney et al., relate to such densification processes and the powders produced therefrom.